Update last_mod times when custom column metadata changes instead of ...

...triggering a backup.

Note that this changes the behavior when a custom column is deleted. Previously deleting a column triggered a full backup, but no other column metadata change did. Now all column metadata changes trigger an update of the last_mod dates.

The new column data will be written to the metadata prefs json file at some point, and will be written to metadata.opf for books modified after the change. These two are sufficient for a correct restore.